desktop: replace Thunar with Nautilus, add File Roller

Thunar didn't integrate cleanly with the GNOME session. Drop the
XFCE file-manager module (thunar/xfconf/exo/tumbler) and restore
Nautilus as the native file manager: un-exclude nautilus, point the
dash favorite at org.gnome.Nautilus.desktop, remove the Thunar
desktop override, and keep gvfs (moved into gnome.nix) for mounting,
trash, and network shares. Add file-roller for archive-manager
integration in Nautilus.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/desktop/desktop-entries.nix

@@ -1,30 +1,6 @@
 { lib, pkgs, ... }:
 
 let
-  thunarDesktopOverride = pkgs.makeDesktopItem {
-    name = "thunar";
-    desktopName = "Files";
-    genericName = "File Manager";
-    tryExec = "thunar";
-    exec = "thunar %U";
-    icon = "org.xfce.thunar";
-    categories = [
-      "Utility"
-      "FileManager"
-    ];
-    mimeTypes = [ "inode/directory" ];
-    keywords = [
-      "folder"
-      "manager"
-      "explorer"
-      "files"
-    ];
-    startupNotify = true;
-    startupWMClass = "Thunar";
-    terminal = false;
-    comment = "Browse files";
-  };
-
   ptyxisDesktopOverride = pkgs.makeDesktopItem {
     name = "org.gnome.Ptyxis";
     desktopName = "Terminal";
@@ -41,7 +17,6 @@ let
 in
 {
   environment.systemPackages = lib.mkAfter [
-    (lib.hiPrio thunarDesktopOverride)
     (lib.hiPrio ptyxisDesktopOverride)
   ];
 }